1. Field of the Invention
The invention relates to a digital phase-frequency detector formed of two flip-flops generating digital output signals received by a charge pump. A feedback path formed of an AND gate and a delay device receives the digital output signals and generates a modified signal received by reset inputs of the two flip-flops.
Such a phase-frequency detector is used in a conventional digital phase-locked control loop. The digital phase-locked control loop includes three function modules, specifically a digital phase detector, an analog loop filter and a voltage-controlled oscillator (VCO) and optionally a frequency divider. The controlled variable in each phase-locked control loop is the phase angle.
The phase angle of a first ac signal is compared with the phase angle of a second ac signal. The phase difference between the two signals is determined in the control system and is used to synchronize the two signals.
In the known phase-locked control loop, one ac input signal, which is a reference signal of a given frequency, is compared with a controlled ac input signal of a variable frequency with the aid of the digital phase detector, which supplies a pulsed output current signal which depends on the phase difference between the two input signals.
The resulting output current signal is integrated over time in the analog loop filter, thus producing an output control voltage which actuates the voltage-control oscillator with regard to its frequency setting. The output signal of the voltage-controlled oscillator is the controlled signal, which is fed via the optionally provided frequency divider as the ac signal to the phase detector as one of the two input signals to be compared with regard to their phase angle.